Pages

Monday, August 27, 2012

After passing Sawtooth dike on the Tuscarora Loop tour (driving ENE on the Midas Road into the Midas Trough), you need to immediatedly look south for the next three miles or so at some spectacular folds in rhyolitic volcanic rocks. The folds are in hills east of the junction of Evans and Clover Creeks (MSRMaps location), with photos beginning at hill 1762, above, and running to the gap southwest of hill 1938. Also see the Midas Area Google Maps view below for specific photo locations.An enlargement of the first photo — with giant copyright notice because my online watermark site once again went down — in case the first photo was too small.Geologic map of the Snowstorm Mountains and vicinity, Elko and Humboldt Counties, Nevada (Wallace, 1993), courtesy U.S. Geol. Survey.

The folds are entirely within an upper Miocene unit mapped by Wallace (1993) as Trpy — younger rhyolite porphyry — which consists of rhyolite and rhyodacite flows and domes.

By the way, I finally figured out how to download useable geologic maps from the USGS website. Here's the map. After downloading, or right-clicking and "Save Target As", one then needs to go here, go to downloads-viewers, and download/install the ExpressView Browser Plug-in (MrSID) for Windows. After installation, then just double-click, or right-click and "Open", the downloaded, saved map file, which has a .sid extension. The map then opens automatically in a useable page in your browser (IE or Firefox). This is so much easier than the old way, when one had to use the command line to open the saved maps. So, ignore these instructions listed as "Help in using this map" on the linked-to USGS map page. Although the instructions might be useful for some purposes, they are unnecessary for making a GEOtiff, tiff, jpg, gif, png, or bmp of a map or some portion thereof. The toolbar that comes with the viewer is a bit obscure; one does some things by right-clicking on tiny icons in the upper left and upper right corners. Saving is accomplished by right-clicking almost anywhere, which brings up options for printing, saving, and other useful items.

The main part of the hills, with some nice folds on the right.

A closer view of those folds.

These last two photos are looking at the northeastern contact area, where the Trpy rhyolite has been faulted against several formations, including the older volcanic rocks of Jake Creek Mountain — rhyolite and rhyodacite flows, domes, and ash-flow tuffs that cap the Owyhee Bluffs to the north — and the older rhyolite porphyry unit (Trpo), which on Owyhee Bluffs is fed, in part, by Sawtooth dike.

First photo, above, is from late spring; photo below is from mid-summer. The cheat grass is partly green and purplish brown in the first photo; everything has turned straw-colored in the second.This general area, from the WSW opening to the Midas Trough to the turnoff to Midas, is a great place to spot antelope:

Antelope in early June.

Antelope in mid-July.

We also spotted a large flock of ravens coming back out of the area in mid-July......and several vultures feeding on roadkill.